This invention relates to a superconductive magnetically levitated railway and a system for feeding power thereto.
An example of the prior art in the technical field to which the present invention appertains is as disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 1-107603.
The arrangement of ground coils and superconducting magnets in a superconductive magnetically levitated railway according to the prior art will now be described with reference to the drawings.
As for the arrangement of the ground coils in the conventional superconductive magnetically levitated railway, ground coils 2 for levitation are laid upon a track bed 1 having a generally U-shaped cross section, as illustrated in FIG. 1, and ground coils 3 for propulsion and guidance are mounted on the side walls of the track bed 1, at a pitch equivalent to an electric angle of 120.degree., in such a manner that the three U-, V-and W-phases will correspond to the N and S poles of a superconducting magnet 4 installed on a vehicle. The left-and-right ground coils for propulsion are null-flux connected so as to form a loop and serve also to guide the vehicle. The number of coils per unit length is such that two coils are used for levitation for each single coil that is used for propulsion.
This arrangement of the ground coils according to the prior art has been investigated to determine how the superconducting magnets are affected by the higher harmonics of the magnetic field generated by the propulsion ground coils disposed in this 120.degree. single-layer configuration. As a result of these investigations, it has been found that the vibration of the inner tank of the superconducting magnets becomes large at a specific frequency and that a large amount of heat loss is produced. Accordingly, a two-layer spaced-pole system has been proposed in which, while such measures as strengthening the inner tank of the superconducting magnet are taken, third harmonics are eliminated, without changing the number of coils, as means for reducing higher-harmonic magnetic fields.
FIG. 2 is a perspective view illustrating the arrangement of the ground coils in the two-layer spaced-pole system mentioned above.
As shown in FIG. 2, a side-wall levitation system is adopted in which ground coils 11 for propulsion and ground coils 12 for levitation and guidance are arranged on the side walls of a track bed 10 having a generally U-shaped cross section.
As shown in FIG. 3, this side-wall levitation system includes ground coils 21 for levitation and guidance and a null-flux line 22. Each levitation-guidance coil 21 has upper and lower coil segments connected together into a figure-eight configuration. The levitation-guidance coils 21 are provided on mountain and ocean sides and are connected together by the null-flux line 22. The levitation-guidance coils 21 are mounted on the side walls of the track bed so that when the center line of a superconducting magnet 20 installed in the vehicle is situated at the center of the levitation-guidance coil 21 having the upper and lower coil segments, the magnetic flux interlinking both coil segments will be the same so that an induced current will not flow through the levitation-guidance coil 21. When the center line of the superconducting magnet 20 installed in the vehicle is situated above or below the center of the levitation-guidance coil 21, an induced current flows through the coil 21 and both the upper and lower coil segments thereof generate a force which returns the superconducting magnet 20 to the center. When the superconducting magnet 20 is below the center, therefore, a levitating force is produced.
Since a levitating force is produced by each horizontal side of the levitation-guidance coil 21, as shown in FIG. 4(a), the same levitating force can be obtained by passing less current through the ground coil 21 in comparison with the system of FIG. 1, and therefore the resistive loss that accompanies levitation also is smaller by comparison. Accordingly, the resistance to traveling of the vehicle (namely the magnetic resistance) caused by magnetism is small. In addition, the magnetic resistance which exhibits its peak value at low velocities can be made zero if the vehicle is made to travel on wheels at the time of low velocity while the height of the center line of the superconducting magnet and the height of the center line of the ground coils are made to coincide.
The guiding force is produced as shown in FIG. 4(b). Specifically, the upper coil segments of the levitating coils on the left and right side walls, as well as the lower coils segments of these levitating coils, are connected by the null-flux line 22 (see FIG. 3) in such a manner that the induced voltages will cancel each other out. As a result, the induced voltage of the levitating coil on the side approached by the superconducting magnet 20 becomes larger than that of the levitating coil on the side from which the superconducting magnet 20 departs, and therefore a circulating current flows through the null-flux line. This causes a repulsive force to be produced on the side approached by the superconducting magnet 20 and an attractive force on the side from which the superconducting magnet 20 departs. Since the upper coil segments and the lower coil segments act in the same manner, a sufficient guiding force is obtained.
In the power-feed system according to the prior art, a current of a frequency commensurate with vehicle velocity and required propulsive force is passed through the propulsion coil of a ground primary-type linear synchronized motor to accelerate and decelerate the vehicle. Furthermore, a dual-feeder system is adopted in which the power supply system is provided in two groups and sections are changed over in succession by feeder section switches as the vehicle advances.
With the ground coil arrangement in the two-layer spaced-pole system, as described above, the mutually adjacent U-, V and W-phase coils of the ground coils 11 for propulsion partially overlap each other in order to eliminate third harmonics without changing the number of coils, as illustrated in FIG. 2. In other words, part of the U-phase coil and part of the V-phase coil are overlapped, part of the V-phase coil and part of the W-phase coil are overlapped, and part of the W-phase coil and part of the V-phase coil are overlapped. The coils are disposed in this manner in successive fashion.
When the coils are arranged in this manner, however, the propulsion ground coils 11 are disposed on the side wall of the U-shaped track bed 10 in such a manner that the V-phase coil 11b is placed upon the U-phase coil 11a, by say of example, and the ground coils 12 for levitation and guidance are disposed on the side wall by being placed upon the propulsion ground coils 11. Accordingly, the ground coils are disposed in three layers, as a result of which overall thickness is increased. This means that a corresponding amount of space is required in the direction of width. In addition, a half-length of a propulsion coil, which is referred to as an end coil, is required to be placed at each break in the guideway beam. Thus, difficulties are encountered in the installation and maintenance of the ground coils.